1. Field of the Invention
This invention relates generally to a control algorithm for a by-pass valve for a thermal sub-system of a fuel cell system and, more particularly, to a control algorithm for a by-pass valve that includes a wax expansion element for a thermal sub-system of a fuel cell system, where the control algorithm provides control and preheating of the wax expansion element using a PDT1 controller, a PI controller and a look-up table.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work can act to operate a vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include two hundred or more individual cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include liquid water and/or water vapor as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
It is necessary that a fuel cell stack operate at an optimum relative humidity and temperature to provide efficient stack operation and durability. A typical stack operating temperature for automotive applications is about 80° C. The stack temperature provides the relative humidity within the fuel cells in the stack for a particular stack pressure. Excessive stack temperatures above the optimum temperature may damage fuel cell components and reduce the lifetime of the fuel cells. Also, stack temperatures below the optimum temperature reduces the stack performance. Therefore, fuel cell systems employ thermal sub-systems that control the temperature within the fuel cell stack to maintain a thermal equilibrium.
A typical thermal sub-system for an automotive fuel cell stack includes a radiator, a fan and a pump. The pump pumps a cooling fluid, such as water and glycol mixture, through cooling fluid channels within the fuel cell stack where the cooling fluid collects the stack waste heat. The cooling fluid is directed through a pipe or hose from the stack to the radiator where it is cooled by ambient air either forced through the radiator from movement of the vehicle or by operation of the fan. Because of the high demand of radiator airflow to reject a large amount of waste heat to provide a relatively low operating temperature, the fan is usually powerful and the radiator is relatively large. The physical size of the radiator and the power of the fan have to be higher compared to those of an internal combustion engine of similar power rating because of the lower operating temperature of the fuel cell system and the fact that only a comparably small amount of heat is rejected through the cathode exhaust in the fuel cell system.
The fuel cell stack requires a certain cooling fluid flow rate to maintain the desired stack operating temperature. The cooling fluid flow rate has to be large enough so that the fuel cell stack does not get hot spots that could damage the cells. Various system parameters determine the cooling fluid flow rate including, but not limited to, the current density of the stack, the cooling fluid temperature, the cooling fluid viscosity, system pressure drop, valve position, etc. For a thermal sub-system employing a centrifugal flow pump, the cooling fluid flow correlates to the system pressure drop because there is no independence of pressure as in displacement pumps.
Because fuel cell systems are thermally sensitive, the cooling fluid flow typically requires a flow controller, such as a proportional-integral (PI) feedback controller, well known to those skilled in the art. Feedback controllers typically require a proportionally controllable pump. Because the pressure is unknown, the actual cooling fluid flow is necessary for the flow controller.
FIG. 1 is a schematic diagram of a thermal sub-system for a fuel cell system 10 including a fuel cell stack 12. A coolant loop pump 14 pumps a suitable cooling fluid, such as a water/glycol mixture, through a coolant loop 16 and cooling fluid flow channels in the stack 12. A first temperature sensor 18 measures the temperature of the cooling fluid in the coolant loop 16 as it is being input into the stack 12 and a temperature sensor 20 measures the temperature of the cooling fluid in the coolant loop 16 as it is being output from the stack 12. A suitable chilling device, such as a radiator 24, cools the cooling fluid in the coolant loop from the stack 12 so that it is reduced in temperature. The radiator 24 may include a fan (not shown) that forces cooling air through the radiator 12 to increase the cooling efficiency of the radiator 24. Further, other cooling devices can also be used instead of the radiator 24. A by-pass line 28 in the coolant loop 16 allows the radiator 24 to be by-passed if the operating temperature of the stack 12 is lower than the desired operating temperature, such as at system start-up. A by-pass valve 30 is selectively controlled to distribute the cooling fluid either through the radiator 24 or through by-pass line 28 to help maintain a desired operating temperature.
Various types of valves are known in the art that can be used for the by-pass valve 30. One known by-pass valve for this purpose is a motorized valve that uses a motor to control the position of the valve to provide the desired temperature of the stack 12. Such motorized valves are fairly good at providing the desired stack temperature because they provide a good proportional movement of the valve mechanism, and provide reliable feedback for a PI controller to establish the position of the valve. However, the valve itself is typically not reliable because it is susceptible to leaks and other mechanical problems. Further, these types of motorized valves are costly, large and heavy.
It is also known in the art to use a two-way valve including a performance-map thermostat having a wax expansion element for the by-pass valve 30. In one particular valve design, a heater element is provided in the wax expansion element that causes it to expand when heated to open the valve and direct the cooling fluid through the radiator 24 in a proportional manner. The density and volume of the wax expansion element changes depending on the temperature of the element. The wax expansion element is designed so that it melts at a certain temperature when heated. The melting temperature of the wax element needs to be in the range of the operating temperature of the cooling fluid, so that the cooling fluid does not cause the wax expansion element to melt. One example of a suitable valve for this purpose is the map-controlled thermostat valve available from Behr Thermot-Tronik GmbH of Kornwestheim, Germany.
FIG. 2 is a simplified diagram of a thermostat by-pass valve 32. The valve 32 includes a wax expansion element 34 having a heater wire 36 therein. When the heater wire 36 is off and the cooling temperature is lower than the melting temperature, the wax expansion element 34 is in its contracted position so that it blocks the flow of the cooling fluid from the radiator 24 to the stack 12 using the seal 66, and allows the cooling fluid flow through the by-pass line 28. The cooling fluid from the by-pass line 28 flows into the valve 32 through opening 60, the cooling fluid from the radiator flows into the valve 32 through the opening 62 and the cooling fluid flows out of the valve 32 to the pump 14 through the opening 64. When the heater wire 36 is on and/or the cooling fluid temperature is higher than the wax melting temperature, so that the wax temperature is higher than the wax melting temperature, the expansion element 34 melts and expands so that the cooling fluid is directed through the radiator 24. The current applied to the heater wire 36 is selectively controlled so that the wax expansion element 34 contracts and expands in a proportional manner to control the amount of cooling fluid that is sent through the radiator 24 as a function of the seals 66 and 68.
A control algorithm is employed to control the heating of the wax expansion element 34 to provide the desired temperature of the stack 12, as discussed above. However, known control algorithms, such as those used for motorized valves, are typically not suitable because of the dynamic nature of the wax element 34, the difficulty to estimate its nonlinear behavior, and no position feedback of the element 34. Particularly, it is difficult to control the deviation of the wax element 34 for long periods of time.
If the temperature of the system is higher than the set-point, a normal controller commands 100% heater power for the heater wire 36. The wax expansion element 34 expands as a result of the heater power and the cooling temperature. If the cooling system performance is too low and the heater controller commands 100%, the maximum displacement of the wax expansion element 34 is reached, so that the wax expansion element 34 gets over-heated. If the temperature set-point decreases, it takes a long time to close the path to the radiator 24 because the wax element 34 is overheated. The result is a large time delay, which causes dynamic problems and stable operation of the stack 12.